1. Field of the Invention
The present invention relates to a transmitting apparatus, a receiving apparatus, and a re-transmission control method, and more specifically to a radio base station and a mobile station in the mobile radio communication system having introduced, for example, the W-CDMA (ŪMTS) communication system.
2. Description of the Related Art
Standardization of the Wideband Code-Division Multiple-Access (W-CDMA, ŪMTS) system, which is one of the third generation mobile communication system, is now under the development with the 3rd Generation Partnership Project (3GPP). As a theme of standardization, the High Speed Downlink Packet Access (HSDPA) is specified to provide a maximum transmission speed of about 14 Mbps for the downlink.
The HSDPA adapts an adaptive modulation and coding (AMC) system which is characterized, for example, in that the QPSK modulation method and the 16-level QAM method are switched adaptively in accordance with the radio communication environment between the base station and mobile station.
Moreover, the HSDPA also adapts the Hybrid Automatic Repeat request (H-ARQ) system. This HSDPA is characterized in that, when a mobile station has detected an error in the data received from the base station, the data is re-transmitted from the base station responding to a request from the mobile station, while the mobile station executes an error correction decoding process using both the already received data and the received data of the re-transmission. In the H-ARQ, as described above, even if an error is detected, the number of times of re-transmission is controlled by effectively utilizing the already received data.
The major radio channels used for the HSDPA include the HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
The HS-SCCH and HS-PDSCH are shared channels in the downlink direction (namely, direction toward a mobile station from a base station), and the HS-SCCH is a control channel for sending various parameters of the data transmitted by the HS-PDSCH. The various parameters, for example, may include the modulation type information which indicates the modulation method used for transmission by the HS-PDSCH, the number of spreading codes assigned (number of codes), and information such as the pattern of rate matching for the transmitting data.
Meanwhile, the HS-DPCCH is a dedicated control channel in the uplink direction (namely, direction toward a base station from a mobile station) and is used to transmit the ACK signal and NACK signal to the base station from the mobile station in accordance with acknowledgment or non-acknowledgment of reception of the data received via the HS-PDSCH. If a mobile station has failed in reception of the data (a CRC error is detected in the receive data or the like), the base station executes the re-transmission control because the NACK signal is transmitted from the mobile station.
Moreover, the HS-DPCCH is also used by the mobile station having measured the receiving quality (for example, the signal-to-interference ratio (SIR)) of the signal received from the base station to transmit the result of measurement to the base station as the Channel Quality Indicator (CQI). The base station determines, in accordance with the CQI received, the environment for the radio communication on the basis of the received CQI. When the communication environment is good, the modulation method is switched to a technique for transmitting the data at a higher speed. If the environment is not good, on the contrary, the modulation method is switched to a technique for transmitting the data at a lower speed (namely, adaptive modulation is executed).
Channel Format
Next, a channel format in the HSDPA will be described below.
FIG. 1 is a diagram illustrating a channel format in the HSDPA. The W-CDMA introduces the code dividing multiplex system and each channel is therefore separated with the spreading code.
The channels not yet described will be described briefly first.
CPICH (Common Pilot Channel) and P-CCPCH (Primary Common Control Physical Channel) are respectively common channels in the downlink direction.
The CPICH is the channel used by a mobile station for estimation of channel condition, searching of cells, and timing reference of the other downlink physical channels in the same cell, and the channel used for transmitting the pilot signal. The P-CCPCH is the channel for transmitting the broadcasting information.
Next, timing relationship of channels will be described with reference to FIG. 1.
As illustrated, one frame (10 ms) is formed of 15 slots in each channel. As described previously, since the CPICH is used as the reference of the other channels, the top of frames of the P-CCPCH and HS-SCCH channels are matched with the top of frame of the CPICH channel. Here, the top of frame of the HS-PDSCH channel is delayed by two slots from the HS-SCCH channel or the like to realize demodulation of the HS-PDSCH channel with the demodulating method corresponding to the received modulation type after the mobile station has received the modulation type information via the HS-SCCH channel. Moreover, the HS-SCCH and HS-PDSCH channels form one sub-frame with three slots.
The HS-DPCCH channel is not matched with the CPICH channel but this channel is provided for the uplink direction and is based on the timing generated in the mobile station.
The channel format of the HSDPA has been briefly described above. Next, the processes up to transmission of the transmit data via the HS-PDSCH channel will be described with reference to the block diagram.
Structure of Base Station
FIG. 2 illustrates a structure of a base station supporting the HSDPA.
In FIG. 2, the reference numeral 1 designates a CRC attachment unit; 2, a code block segmentation unit; 3, a channel encoder; 4, a bit separating unit; 5, a rate matching unit; 6, a bit collecting unit; 7, a modulator.
Next, operations of each block will be described.
The transmitting data transmitted via the HS-PDSCH channel (data accommodated within one sub-frame of the HS-PDSCH channel in FIG. 1) is first subjected to the CRC arithmetic process in the CRC attachment unit 1 and the result of arithmetic operation is added to the last part of the transmitting data. The transmitting data to which the result of CRC arithmetic operation is added is then input to the code block segmentation unit 2 and is then segmented into a plurality of blocks. This process is required to shorten the data length in units of the error correction encoding, considering the load of decoding process in the receiving side. When the data length exceeds the predetermined length, the code block is equally segmented to a plurality of blocks. An integer 2 or larger may be selected as the number of segmentations but the number of segmentations 2 may be selected to simplify the description. If the data length is rather short, segmentation of blocks is unnecessary.
The segmented transmitting data are respectively processed as the object data of the individual error correction encoding process in the channel encoder 3. In other words, the error correction encoding process is respectively executed for the segmented first block and second block. As an example of the channel encoding process, a turbo encoding process may be listed.
Here, the turbo encoding process will be described briefly. In the turbo encoding process, when the data as the object of the encoding process is defined as U, the data U itself, the data U′ obtained by the convolutional encoding of the data U, and the data U″ obtained by the convolutional encoding of the data U after the interleave (re-arrangement) process of the data U may be output. Here, the data U is called the systematic bits and can be understood, in the turbo decoding process, as the data used in two element decoders and the data having a higher degree of importance because the application frequency is high. On the other hand, the data U′, U″ are parity (redundant) bits. These bits are data used only in one of the two element decoders and can be understood as the data having a degree of importance which is lower than that of the data U because the application frequency is low.
Namely, it can be said that since the systematic bits have the higher degree of importance than that of the parity bits and the systematic bits are received more correctly, a more accurate decoding result can be obtained with the turbo decoder.
The systematic bits and parity bits generated as described above are input as serial data to the bit separating unit 4 and this bit separating unit 4 separates the input serial data into the data U, U′, U″ of three systems and then outputs these data as parallel data.
The rate matching unit 5 performs the puncture process for deleting the bits with the predetermined algorithm and also executes the repetition process to repeat the bits in order to store the data within the sub-frame formed of three slots of the HS-PDSCH channel.
As described above, the bits having completed the bit adaptation process to the sub-frame are then input in parallel to the bit collecting unit 6.
The bit collecting unit 6 generates bit sequences wherein each bit sequence including four bits indicating one signal point, for example, of 16-level QAM modulation based on the input data, and then outputs these bit sequences. At the time of generation of bit sequences, the systematic bits are preferably arranged, for the first transmission, in the side of upper bits in which an error is not easily generated.
The modulator 7 outputs the signal of the 16-level QAM modulation to provide the amplitude and phase corresponding to the signal points indicated with the input bit sequence and then transmits the signal to the antenna (not illustrated) after conversion to radio frequency through frequency conversion.
The HSDPA is well-known and is disclosed, for example, in the Japanese Published Unexamined Patent Applications Nos. 9741/2002 and 281003/2002, and in the non-patent document “3G TS 25. 212” (3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD)).
According to the background technology described above, when a base station receives the NACK signal from a mobile station, the base station executes a re-transmission. However, in this case, the re-transmission is made in using transmission power similar to that of the first transmission because a particular control in which the re-transmission power is different from that of the first transmission is never performed.
However, as described previously, when the received data is reproduced (decoded) through combining of both the signal received first and the signal received by the re-transmission, the combined gain is obtained.
Accordingly, the re-transmission is executed with a transmission power similar to that of the first transmission even when it is not required in order to correct the error that occurred in the first transmission. This means that the re-transmission is performed with excessive quality.
Therefore, a need arises for a technique to control excessive quality in the receiving apparatus by selectively controlling the re-transmission for the first transmission.